Transistor gates are used in a variety of different types of semiconductor devices. A number of different types of conductive materials have been used to form the elements, or lines, of transistor gates. Tungsten silicide is an example of such a conductive material, and is widely used in semiconductor devices that employ current state-of-the-art technologies.
As feature dimensions of semiconductor device structures decrease, limitations on the materials that are used therein become ever more apparent. For example, at line widths of about 55 nm and less, the bulk electrical resistance of tungsten silicide (WSix) increases to about 175μΩ to about 250μΩ. Bulk electrical resistances of this magnitude are undesirable, as they decrease the rate at which conductive lines transmit electrical signals and may cause the conductive lines and other elements of a semiconductor device to be heated to temperatures that may adversely affect the performance, structural integrity, and reliability of the semiconductor device.
Cobalt disilicide (CoSi2) has a much lower bulk resistance than tungsten silicide (e.g., about one tenth or less of the bulk resistance of tungsten silicide), but does not retain its quality when exposed to high temperatures, as are encountered during many semiconductor device fabrication processes. For example, the epitaxial processes that are required to form “raised” (i.e., at the same elevation or a higher elevation than a gate oxide) source and drain regions adjacent to transistor gates of some types of transistors, such as those of dynamic random access memory (DRAM) devices and NAND flash memory devices, require temperatures of 900° C. or more. As a consequence of the relatively low thermal stability of cobalt disilicide, it could not be used in transistors with raised source and drain regions or other structures in which conductive elements have conventionally been fabricated before all high temperature processes have been completed.
Accordingly, there is a need for processes that facilitate the use of lower bulk resistance conductive materials in the fabrication of conductive features of semiconductor devices with ever-decreasing feature dimensions, as well as for semiconductor device structures with conductive features that are formed with low bulk resistance materials.